Objects can be created in a limited space with the assistance of generative methods according to computer data. For example, particle material can be deposited layer by layer through laser sintering on a building platform determined by the working field, and then by means of selectively applied laser radiation selectively hardened. The building platform is subsequently lowered and the procedure of applying another layer of particle material and subsequent hardening is repeated until the mould has been formed to its full dimensions.
In other methods the hardening of the particle material is achieved by the selective application of a binding agent to the layers of particle material.
In order to prevent the loose particle material of both described methods from flowing uncontrollably over the edges of the working field, in each case the building platform is enclosed within a container. The maximum size of the components can therefore only be that of the container. Bigger components must be put together from several parts, at the risk of a loss of precision. With the production of components that are significantly smaller than the container, the whole cross section surface of the container must be filled with particle material, so that the support effect of the particle material stays intact for the incomplete laminated compound. As a rule this results in a greater utilization of excess material relative to the size of the component as in the case of bigger components.
For example, it is known from EP 0 644 809 B1 how to construct a ceramic casting mould with several casting cavities in a shallow container, which encloses a rectangular working field. For a linear depositing of the particle material on the floor of the container, and the following layers, a longitudinal dispensing device is envisaged. Together with an elongated print head with a row of selectively operated print nozzles for the binding agent, the dispensing device is moved above the container on tracks running on both sides of the container. However, in this process the distance between the dispensing device and the print head on the one hand and the top of the already built-up part of the laminated compound in the container on the other hand, is reduced by the thickness of the new layer at the depositing of every new layer. This results in different surface coating conditions for the different layers of the laminated compound.
According to another design example, from EP 0 644 809 B1, a construction container with a building platform as container floor, which is lowered by the thickness of the layer before the depositing of each layer, is envisaged. Generally, the container is lowered into a work opening of a workbench in such a way that the top of the container is aligned with the workbench. The thickness of each layer is determined by the cavity between the top of the container and the previous layer resulting from the lowering of the container. The particle material is poured into the cavity by a surface coating unit that can be moved above the workbench, and it is smoothed along the top of the workbench by a cylinder and/or a scraper. Excess particle material is pushed away onto the workbench and disposed of.
The provision of a construction container, which contains a building platform that can be lowered in a vertical direction, requires a large measure of technical complexity in the sealing of the container wall against the building platform, to prevent the uncontrolled flow-out of particle material through the gap between the edge of the building platform and the container wall, without running the risk that the platform gets jammed against the container wall due to the granular particle material.
A further disadvantage of the currently specified construction forms, with building platforms that can be lowered, lies in the continuous increase of the weight that has to be moved around on the building platform as the building process proceeds. In applying the new layer, in particular, it can be necessary that the powder bed be lowered by slightly more than the actual thickness of the layer and then be lifted again to the required measurement to be able to set the thickness of the layer accurately enough. With this revolving operation not only the total weight of the powder fill including the building platform but also the friction force of the powder bed against the container wall and the friction of the seal between the building platform and the container wall must be overcome. This leads to extreme pressure on the tracks and power unit.
According to another design example, from EP 0 644 809 B 1, one container wall is discarded and instead an artificial container wall is thereby constructed in the work opening of the workbench around the laminated compound, so that the particle material of every newly deposited layer that is supported by the edge of the work opening is also hardened along the perimeter edge of the newly deposited layer. With such a design form the sealing problem_between the building platform and the construction container wall is indeed prevented; however, here too the complete available working field must be filled with particle material. Furthermore, a sealing problem develops between the edge of the work opening and the already hardened part of the artificial container wall at the completion of the previous layer, over which the particle material of the newly deposited layer extends.
Concerning this, another design example is known, from EP 0 644 809 B1. With this design form an artificial container wall is constructed. The particle bed is not lowered relative to the workbench in which the work opening is formed, from which edge every newly deposited layer is enclosed. Instead, the workbench with the surface coating unit and the print head for the depositing of the particle material and the binding agent is lifted relative to the particle bed. This design example is considered especially suitable for massive components, for which construction it is easier to lift the workbench with the surface coating and print mechanism instead of lowering the powder bed.